feat(panel): show scroll-to-bottom control when not at latest

[zzalancelot]

Summary

• Show a floating downward chevron at the bottom-right of the conversation pane when the user is not near the latest messages.
• Clicking it smooth-scrolls back to the bottom.
• Auto-follow new messages only while already near the bottom, so scrolling up to read is not interrupted.

Test plan

• Reload the unpacked extension from ~/.dsh/browser-extension
• Open a long conversation and scroll up: the bottom-right chevron appears
• Click the chevron: the pane smooth-scrolls to the latest messages and the button hides
• Stay at the bottom during streaming: the pane keeps following new content
• Scroll up during streaming: the pane does not jump; the chevron remains available
• pnpm --filter dsh-browser-extension exec vitest run tests/scroll-bottom.spec.ts tests/panel-styles.spec.ts

Confidence Score: 5/5

The PR appears safe to merge, with the previously reported stale bottom-state issue resolved by observing the actual scrollport.

Summary

• Tracks whether the conversation is near its bottom and only follows new content in that state.
• Shows a localized floating control for returning to the latest messages.
• Observes scrollport resizing so surrounding panel UI cannot leave the bottom state stale.
• Adds focused tests for threshold calculations and the floating-control layout.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Conversation geometry or scroll changes] --> B[Measure distance from bottom]
  B --> C{Within 48 pixels?}
  C -->|Yes| D[Keep following new messages]
  C -->|No| E[Stop automatic scrolling]
  E --> F[Show scroll-to-latest control]
  F -->|User clicks| G[Smooth-scroll to bottom]
  G --> D

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